1. Field of the Invention
The present invention relates to an improvement in a silent chain power transmission apparatus for use as a timing chain for timed power transmission in an automobile, a power transmitting chain in an industrial machine, or the like.
2. Description of the Related Art
In a conventional silent chain power transmission apparatus, an endless silent chain is employed which comprises a multiplicity of link plates interleaved by a multiplicity of connecting pins in a fingers-crossed fashion, as illustrated in FIG. 8 hereof. Each link plate 100 has a pair of V-shaped teeth 10 each defined by inside and outside faces 1, 2 for meshing with teeth of a sprocket S as shown in FIG. 8 hereof. As can be clearly seen from FIG. 7, the inside tooth faces 1 are scooped out deeper inwardly than the profiles, symmetrical with the profiles of the outside tooth faces 2, of imaginary inside tooth faces 1xe2x80x2 so that their interference with the teeth of the sprocket upon intermeshing can be prevented. Accordingly, pitch line Li of the inside tooth faces is positioned closer to the pin center line Lp than a pitch line Lo of the outside tooth faces. The pitch line Li is a line passing parallel to pin center line Lp over two inside tooth faces 1 of the link plate at points where a distance between those two points becomes 1/2 of the chain pitch 2, which is equal to the distance between two pins of the link plate. Similarly, the pitch line Lo of the outside tooth faces is a line passing parallel to the pin center line Lp over the two outside tooth faces 2 at points where the distance between the two points becomes 3/2 of the chain pitch P.
More specifically, the profiles of the V-shaped link teeth 10 of the Link plate 100 are interrelated to each other to satisfy Hixe2x89xa6Ho, where Hi is a distance from the pin center line Lp to the pitch line Li of the inside tooth faces and Ho is a distance from the pin center line Lp to the pitch line Lo of the outside tooth profiles.
Though not shown, the sprocket S to be intermeshed with the silent chain C has teeth of profiles formed in a hobbing process using a hob cutter.
It has been known that when the silent chain arranged as explained above is brought into meshing engagement with the sprocket S, the silent chain C is wound around the sprocket in a polygonal manner and used practically in such manner.
However, since the link plates 100 of the silent chain C mesh at their outside tooth faces alone with the teeth of the sprocket S, the silent chain makes, upon commencement of and release from meshing engagement with the sprocket, a polygonal motion involving up and down movements radial of the sprocket.
A quantitative analysis of the polygonal motion may be made. As can be appreciated from FIG. 8, since a pitch angle of the silent chain C and the number of teeth N of the sprocket S have relations expressed by xcex1=(360xc2x0/N), the followings may be established.
First, when the chain pitch is P and the center of the sprocket S, the connecting pin 200a and a free-span part of the chain C following the link plate 100b are arranged to form a right angle, the distance from the center of the sprocket S to the connecting pin 200a on the free-span part of the chain becomes U=P/2 sin(xcex1/2). The term xe2x80x9cfree-span part of the chainxe2x80x9d used herein represents a chain portion not completely engaged with the sprocket but being in a process of engaging with the sprocket. In other words, the free-span part of the chain is that part of the chain which is not completely constricted by the sprocket and is thus placed in a relatively free state.
Next, as shown in FIG. 9, when the sprocket S is rotated half a pitch angle xcex1/2 and the centers of the sprocket S and the link plate 100b and the free-span part of the chain are positioned to form a right angle, the distance from the center of the sprocket S to the pitch line of the free-span part of the chain is expressed by V=P/2 tan(xcex1/2).
As can be readily appreciated from the above discussion, the silent chain C just started meshing with the sprocket S makes a polygonal motion of amplitude Hs=Uxe2x88x92V involving up and down movements during half pitch angle (xcex1/2) rotation of the sprocket S.
Consequently, in the thus-arranged conventional silent chain power transmission apparatus, the polygonal motion of amplitude Hs=Uxe2x88x92V and associated up and down movements cause the link plates 100a, 100b, 100c to hit the teeth of the sprocket S, resulting in an undesired motion. In addition, when the silent chain apparatus is operated at a high speed, undesired vibratory and impact sounds are produced at the start of meshing of the silent chain with the sprocket. Further, impact of the chain C against the sprocket S shortens the lives of the chain and sprocket. Moreover, the polygonal motion causes the tension of the chain to change, thereby further decreasing durability of the silent chain.
To this end, an improved silent chain power transmission apparatus has been proposed in Japanese Patent Laid-Open Publication No. HEI-8-184348 in which arrangements are made to reduce vibrations, sounds and chain tension fluctuations arising owing to the polygonal motion of the chain. The disclosed apparatus comprises a silent chain having rows of interleaved link plates with teeth defined by inner and outer flank surfaces, and a sprocket having teeth for meshing with the link plate teeth. The sprocket is gear-cut by using a rack cutter having teeth of profiles identical to the profiles of the link plates upon linear stretching of the silent chain. As a result, the silent chain can mesh with the sprocket teeth smoothly, thereby minimizing polygonal motion of the chain and thus reducing noisy impact sounds.
In the thus-arranged silent chain power transmission apparatus, upon commencement of intermeshing, since the link plates of the silent chain come into meshing engagement with the sprocket in a similar manner as the tooth profiles of the rack cutter, the chain polygonal motion can be minimized. However, when the link plates of the silent chain wind around and get seated on the sprocket, the chain polygonal motion arises as in the conventional power transmission apparatus. Thus, it is not possible for the proposed power transmission apparatus to completely kill the chain polygonal motion throughout the course of meshing of the chain with the sprocket. Consequently, vibrations, noisy sounds and chain tension fluctuations produced in the proposed power transmission apparatus cannot be reduce to substantially zero.
Moreover, due to the polygonal motion of the chain, a concave bottom surface defined between the opposed inner flanks or tooth faces of each link plate interferes with the edges of the teeth of the sprocket. Similarly, the edges of the V-shaped teeth of the link plates interfere with the root bottoms of the sprocket. As a result, partial wear and breakage are inevitable at the interfering parts of the chain and sprocket.
It is therefore an object of the present invention to provide a silent chain power transmission apparatus which can prevent up and down movements of a silent chain thereof, which are due to polygonal motion of the chain when the chain intermeshes with, and becomes seated on, a sprocket, and thereby substantially reduce vibratory and impact sounds and fluctuation in tensioning of the chain, and which can prevent wear of concave bottom surfaces of link plates forming the chain, edges of V-shaped teeth of the link plates and the root bottom of the sprocket and thereby improve durability of the apparatus.
According to the present invention, there is provided a silent chain power transmission apparatus which comprises: an endless silent chain comprised of a multiplicity of link plates interleaved by a multiplicity of connecting pins, each of the link plates having a pair of V-shaped teeth and a pair of pinholes for fitting the connecting pins, each of the V-shaped link teeth having opposed inside tooth faces defining insides thereof and outside tooth faces defining outsides thereof; and a sprocket having a plurality of involute teeth for intermeshing with the V-shaped teeth of the link plates, the inside tooth faces having profiles identical to tooth profiles arranged axially of a hob cutter for forming the involute teeth of the sprocket, the inside and outside tooth faces being profiled to satisfy the expression Hi=Ho+Hs, where Hi is a distance from a pin center line, passing over the centers of the connecting pins, to a pitch line of the inside tooth faces, Ho is a distance from the pin center line to a pitch line of the outside tooth faces, and Hs is an amplitude of polygonal motion of the chain, each of the link plates having a concave bottom surface continuing from and defined between the opposed inside tooth faces at a position where interference of the concave bottom surface with corresponding tooth edges of the involute teeth, which arises owing to the chain polygonal motion amplitude when the outside tooth faces of the link plate are brought into meshing contact with the involute teeth and get seated thereon, can be avoided.
In a preferred form, the V-shaped teeth have tooth edges provided at a position where interference of the tooth edges with concave bottom surfaces defined between opposed involute teeth of the sprocket can be avoided. The interference arises owing to the chain""s polygonal motion amplitude when the outside tooth faces of the link plate are brought into meshing contact with the opposed involute teeth and become seated thereon.
The inside tooth faces of the silent chain may have any profiles as long as they are profiled to conform to the axially-arranged tooth profiles of the hob cutter for hobbing the sprocket. They may have linear tooth profiles or curved tooth profiles. The hob cutter used to form the involute teeth on the sprocket has linear tooth profiles. Other hob cutters having special curved tooth profiles may also be used depending on the tooth profiles of the sprocket.
Similarly, the outside tooth faces of the silent chain may have any profiles as long as they satisfy Hi=Ho+Hs. They may have linear or curved tooth profiles, or a combination thereof. They may be formed by punching a link plate or profiled in a shaping process such as sintering.
It is essential that the concave bottom surface of the link plate be formed at a position where its interference with the edges of the sprocket teeth can be avoided. The interference arises owing to the amplitude of the chain""s polygonal motion upon meshing engagement of the outside tooth faces of the link plate with the sprocket. If the concave bottom surface is provided at a position excessively far away from the position where such interference can be avoided, then the distance from the concave bottom surface to the rear part of the link plate and the distance from the pinholes become small, thereby decreasing strength of the link plate. Thus, the concave bottom surface should be provided at a position as close as possible to the position where such interference can be avoided.
Since the concave bottom surface of the link plate has an arcuate profile, stress applied to the root bottom surface upon power transmission can be uniformly dispersed. This leads to the advantage that decrease in strength of the link plate can be prevented.
It is also essential that the edges of the V-shaped link teeth are profiled at a position where their interference with the root bottoms of the sprocket, which arises owing to the amplitude of the chain polygonal motion when the outside tooth faces of the link plate get engaged with the teeth of the sprocket and seated on the latter, can be avoided. Such link tooth edges may have linear tooth profiles or curved tooth profiles, or a combination thereof.
In operation of the silent chain power transmission apparatus of the present invention, the V-shaped teeth formed on the link plates of the silent chain starts to mesh with teeth of the sprocket at the respective inside tooth faces and they are seated on the sprocket via meshing engagement between the outside tooth faces and the sprocket teeth as the link plates travel around the sprocket.
In this instance, since the inside tooth faces have profiles identical to tooth profiles arranged axially of a hob cutter for forming the teeth of the sprocket, the inside faces of the link plates, coming in succession from the tangential direction of the sprocket in the same manner as the hob cutter, are accepted by the sprocket without resistance and can smoothly start to mesh with the sprocket teeth. This enables the distance from the free-span part of the chain to the sprocket center to be maintained always constant.
By the relation Hi=Ho+Hs established between the inside and outside tooth faces, the inside tooth faces of the link plates approaching the sprocket in the tangential direction project outward from the outside tooth faces of the overlapping preceding link plates. Even when the sprocket turns half a pitch angle to cause polygonal motion of the chain with an amplitude Hs in the course from the start of mesh of the inside tooth faces to the seating of the outside tooth faces relative to the same sprocket tooth, the inside tooth faces projecting outward from the outside tooth faces preferentially contact the sprocket tooth, so that the distance from the center of the sprocket to the centers of pin holes in the link plates can be always kept constant. This eliminates up and down movements of the silent chain with respect to the sprocket.
In addition, the concave bottom surface of the link plate is provided at a position where its interference with the corresponding tooth edge of the sprocket, which results from the amplitude Hs of the polygonal motion of the chain, can be avoided. This enables the link plates, approaching the sprocket in the tangential direction, to wind around the tooth edges of the sprocket without contacting the concave bottom surfaces defined between the teeth of the link plates. Further, at the seated position, upon rotation through half a pitch angle, the concave bottom surface of the link plate 100a subsides toward the center of the sprocket S by the amplitude of the chain polygonal motion. This ensures that the outside tooth face of the link plate is brought into a seated position on the sprocket.
Furthermore, the tooth edge of the V-shaped link tooth has its profile positioned so that its interference with the confronting root bottom of the sprocket, which results from the amplitude Hs of the polygonal motion of the chain, can be avoided. As a result, the link plate traveling toward the sprocket in the tangential direction winds around the sprocket with its tooth edge kept away from the root bottom. Upon seating of the link plate on the sprocket as a result of its advance by half a pitch angle, the tooth edge of the link plate sinks toward the center of the sprocket by a distance corresponding to the amplitude Hs of the polygonal motion of the chain, thereby ensuring that the outside tooth faces of the link plate are seated on the sprocket.